docs: Link to latest release documentation on Swift Package Index (#124)

Motivation
----------

By default, Swift Package Index shows the documentation for the latest
release. In release 1.0.0, `containertool` was still the first target in
`.spi.yml`, which caused the left-hand navigation bar (the curation) to
show the topics for `containertool` even though the main content is the
Swift Container Plugin Documentation.

As a temporary work-around, #112 changed links from README.md to point
to SPI's documentation for the `main` branch. This caused the updated
`.spi.yml` to be used and the correct curation to be shown, but also
meant that unreleased features could not be documented because the live
documentation would diverge from what had been released.

Version 1.0.1 has now been released, with some refactoring and
documentation changes. Links in `README.md` can now safely refer to the
most recent release again.

Fixes #113, reverts changes in #112.

Modifications
-------------

* Links from `README.md` lead to documentation describing the most
recent release, not the live state of `main`.

Result
------

* Users will see the documentation for the latest release, with the
correct curation panel.
* Unreleased features can be documented on `main` without the risk of
confusing users looking for documentation for the latest release.

Test Plan
---------

No functional change.   Links checked manually.

Author: euanh

README.md

@@ -1,6 +1,6 @@
 # Swift Container Plugin
 
-[![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin)
+[![](https://img.shields.io/badge/docc-read_documentation-blue)](https://swiftpackageindex.com/apple/swift-container-plugin/documentation)
 [![](https://img.shields.io/github/v/release/apple/swift-container-plugin?include_prereleases)](https://github.com/apple/swift-container-plugin/releases)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-container-plugin%2Fbadge%3Ftype%3Dswift-versions)](https://swiftpackageindex.com/apple/swift-container-plugin)
 [![](https://img.shields.io/endpoint?url=https%3A%2F%2Fswiftpackageindex.com%2Fapi%2Fpackages%2Fapple%2Fswift-container-plugin%2Fbadge%3Ftype%3Dplatforms)](https://swiftpackageindex.com/apple/swift-container-plugin)
@@ -14,16 +14,16 @@ Container images are the standard way to package cloud software today.   Once yo
 
 Use Swift Container Plugin to build and publish container images for your Swift services in one streamlined workflow with Swift Package Manager.
 
-![Swift Container Plugin flow diagram](Sources/swift-container-plugin/Documentation.docc/_Resources/swift-container-plugin-flow-diagram.png)
+[![Swift Container Plugin flow diagram](Sources/swift-container-plugin/Documentation.docc/_Resources/swift-container-plugin-flow-diagram.png)](https://swiftpackageindex.com/apple/swift-container-plugin/documentation)
 
-1. [Add the plugin to your project's dependencies](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin/adding-the-plugin-to-your-project) in `Package.swift`.
-2. [Build and package your service](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin/build) using Swift Package Manager.
-   - If you are building on macOS, [use a Swift SDK](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin/requirements) to cross-compile a Linux executable.
+1. [Add the plugin to your project's dependencies](https://swiftpackageindex.com/apple/swift-container-plugin/documentation/swift-container-plugin/adding-the-plugin-to-your-project) in `Package.swift`.
+2. [Build and package your service](https://swiftpackageindex.com/apple/swift-container-plugin/documentation/swift-container-plugin/build) using Swift Package Manager.
+   - If you are building on macOS, [use a Swift SDK](https://swiftpackageindex.com/apple/swift-container-plugin/documentation/swift-container-plugin/requirements) to cross-compile a Linux executable.
    - If you are building on Linux, use your native Swift compiler to build a Linux executable.   If you have special requirements such as building a static executable, or cross-compiling to a different processor architecture, use a suitable Swift SDK.
 3. The plugin automatically packages your executable in a container image and publishes it to your chosen container registry.
-4. [Run your container image](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin/run) on any container-based platform.
+4. [Run your container image](https://swiftpackageindex.com/apple/swift-container-plugin/documentation/swift-container-plugin/run) on any container-based platform.
 
-Find full details [in the documentation](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin).
+Find full details [in the documentation](https://swiftpackageindex.com/apple/swift-container-plugin/documentation).
 
 ## Usage
 
@@ -75,11 +75,11 @@ Trying to pull registry.example.com/myservice@sha256:a3f75d0932d052dd9d448a1c904
 * A container runtime is not required to build an image, but one must be available wherever the image is to be run.
 
 ### Find out more
-* Learn more about setting up your project in the [plugin documentation](https://swiftpackageindex.com/apple/swift-container-plugin/main/documentation/swift-container-plugin).
+* Learn more about setting up your project in the [plugin documentation](https://swiftpackageindex.com/apple/swift-container-plugin/documentation/swift-container-plugin).
 
 * Take a look at [more examples](https://github.com/apple/swift-container-plugin/tree/main/Examples).
 
 * Watch some talks:
 
     * [How to put Swift in a box](https://fosdem.org/2025/schedule/event/fosdem-2025-5116-how-to-put-swift-in-a-box-building-container-images-with-swift-container-plugin/) at [FOSDEM 2025](https://fosdem.org/2025/schedule/track/swift/).
-    * [Swift to the cloud in a single step](https://www.youtube.com/watch?v=9AaINsCfZzw) at [ServerSide.Swift 2024](https://www.serversideswift.info/2024/speakers/euan-harris/).
+    * [Swift to the cloud in a single step](https://www.youtube.com/watch?v=9AaINsCfZzw) at [ServerSide.Swift 2024](https://www.serversideswift.info/2024/speakers/euan-harris/).